The twentieth century, which is known as the era of mass consumption and mass disposal, has come to an end, and in the twenty-first century where establishment of an environmentally friendly society is demanded, as the problem of depletion of fossil resources and the problem of global warming are becoming more serious, promotion of the utilization of biomass resources, which are recyclable resources, is under expectation.
Currently, among the biomass resources, production of bioethanol using sugar cane or corn as a starting material is in active progress in the United States, Brazil and the like. This is because sugar cane or corn contains a rich content of sucrose or starch, and accordingly, it is easy to prepare a sugar solution therefrom for fermentation. However, sugar cane and corn are originally foodstuffs, and when these are used as starting materials, there is a serious problem that a competition occurs between the usage as the starting material and the usage as foodstuffs or feedstock, causing an increase in the starting material price. Thus, development of a technology to use a non-edible biomass as a starting material is under way.
Examples of the non-edible biomass include cellulose that is present most abundantly on Earth, and most of cellulose exists in the form of a polysaccharide-based biomass which is a complex of cellulose with lignin or a hemicellulose, which is an aromatic polymer. A technology of producing a monosaccharide or an oligosaccharide of a pentose or a hexose from cellulose or hemicelluloses in a polysaccharide-based biomass, fermenting the obtained monosaccharide or oligosaccharide, and converting the fermentation product to various compounds originating from a polysaccharide-based biomass, such as ethanol or lactic acid, is attracting public attention. However, as described in Technologies Utilizing Biomass Energy, reviewed by Yukawa, Hideaki, CMC Publishing, Inc, (2006), a polysaccharide-based biomass is a complicated construct of cellulose, hemicelluloses and lignin, and cellulose or hemicelluloses are protected by lignin from being subjected to biodegradation, so that the composition ratios vary in a wide range depending on the regional and seasonal conditions and the starting material. For this reason, it is not easy to selectively pick out only a monosaccharide or an oligosaccharide of a pentose or a hexose.
Investigations have hitherto been made on a pretreatment method of destroying or softening the protective walls of lignin by treating a polysaccharide-based biomass using an acid, an alkali, an enzyme, subcritical water (supercritical water) or the like, and recovering a liquid or solid containing a monosaccharide or an oligosaccharide of a pentose or a hexose. For example, since a treatment based on subcritical water (supercritical water) has a short treatment time, and does not require a mineral acid or the like, that is, does not require a neutralization treatment, the treatment is advantageous from an environmental aspect such that a side product such as plaster is not generated. Thus, this treatment is attracting attention as a next-generation treatment method of environmentally conscious type. However, as described in JP-A-2005-270056, since subcritical water (supercritical water) is highly reactive, there are difficulties in controlling the reactivity, and various fermentation inhibitors such as furfural and 5-hydroxymethylfurfural, which are overdegradation products of sugars, as well as vanillin and guaiacol, which are lignin-derived aromatic compounds, are also generated at the same time, so that the treatment product cannot be directly used in the fermentation step. Furthermore, according to the pretreatment conditions, the concentration of the obtainable monosaccharide or oligosaccharide of a pentose or a hexose may be low, and in this case, it is necessary to carry out simple concentration of the monosaccharide or oligosaccharide to about several-fold to ten-fold before supplying the monosaccharide or oligosaccharide to the fermentation process. At this time, while the monosaccharide or oligosaccharide of a pentose or a hexose is concentrated, the fermentation inhibitors are also concentrated at the same time, so that it is difficult to use the concentrate in the fermentation process.
In regard to such problems, investigations are being made on the removal of fermentation inhibitors. For instance, Biotechnology Letters, Vol. 5, No, 3, pp. 175-178 (1983) discloses a method of removing a fermentation inhibitor through adsorption to activated carbon. However, this method has a problem that since the activated carbon adsorbs not only fermentation inhibitors but also monosaccharides or oligosaccharides of pentoses or hexoses, the yield of the monosaccharides or oligosaccharides of pentoses or hexoses is decreased.
JP-A-2005-270056 discloses a method of removing fermentation inhibitors through adsorption to wood-based carbon, and in this method, since fermentation inhibitors can be selectively adsorbed and removed, a monosaccharide or an oligosaccharide of a pentose or a hexose can be obtained with a good yield. However, since the removal mechanism involves adsorption, if the adsorption capacity is saturated, the fermentation inhibitors run off and contaminate the apparatuses, pipes and the like in the subsequent steps. Unless the fermentation reaction is carried out accurately, high quality products cannot be obtained, and especially in the case of carrying out the production by continuously operating the apparatuses while continuously supplying the starting materials, a method of stably and certainly removing fermentation inhibitors is desired, because the occurrence of contamination of apparatuses, pipes and the like brings on an increase in the cost and a decrease in the product quality. Furthermore, in the case of using a starting material having a low concentration of a monosaccharide or oligosaccharide of a pentose or a hexose, a method capable of reducing two steps, namely, a step for the concentration of a monosaccharide or an oligosaccharide of a pentose or a hexose and a step for the removal of fermentation inhibitors, into one step, or reducing the burden of the concentration step, is desired from the viewpoint of reducing the cost and enhancing the product quality.
On the other hand, in the case of using construction waste materials such as plywood as a polysaccharide-based biomass, acetic acid, formic acid and the like originating from the adhesive contained in the plywood act as fermentation inhibitors. There, JP-A-2004-187650 discloses a method of removing volatile fermentation inhibitors such as acetic acid and formic acid by distillation. This method is barely effective only when the non-volatile fermentation inhibitors that cannot be removed by distillation are present at a concentration that does not have adverse effects on the fermentation process, and it is difficult to apply the method when a polysaccharide-based biomass having a broad composition range is used as a starting material.
It could therefore be helpful to provide a method of producing a compound originating from a polysaccharide biomass by stably and certainly removing fermentation inhibitors that serve as an obstacle to reduce the burden of and to promote streamlining of at least one of the following steps, that is, a step for producing a monosaccharide and/or an oligosaccharide of a pentose and/or a hexose using a polysaccharide-based biomass having a broad composition range as a starting material, and a step for converting the monosaccharide and/or oligosaccharide thus obtained into a chemical via fermentation.